Part of that (the cut placement) is making the prosthetics though. Yeah, you can make good looking cuts elsewhere, but the more the site moves, the more hassle it is. Facial prosthetics in particular run into trouble when they're near the mouth or brows. So you end up with cheeks being preferred overall.

With the body, you run into issues with making something visible, movable, and believable. Gashes across the abs fail at all three, whereas shoulder and pecs are solid choices. Legs would be too, but they're not in frame as much, and shouldn't be without a good reason.

Everything on screen is a compromise of some sort.